Absorption and luminescence spectra and energy levels of Nd3+ and ...

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crystallizes in the trigonal system and contains the spatial group C6 -R3C; Li and Nb5+ occupy the equivalent rhombohedra1 vacancies with local C3 symmetry ...

Short Notes

K37

phys. stat. sol. (a) 3, K37 (1970) Subjtct classification: 20.1 and 20.3; 13.4; 22.8.1 Institute of Crystallography, Academy of Sciences of the USSR, Moscow Absorption and Luminescence Spectra and Energy Levels of Nd

3+

and E r

3+

Ions

in LiNb03 Crystals BY V. T. GABRIELYAN, A.A. KAMINSKII, and L. LI Dedicated to Prof. Dr. Dr. h.c. P. GORLICH, on the occasion of his 65th birthday We studied LiNbOQcrystals activated by Nd3+ and E r

3+

ions. The object of

study was twofold: namely, to analyze the activator centers formed in this crystal and to obtain fuller information on the scheme of ionic levels. Lithium metaniobate I+

crystallizes in the trigonal system and contains the spatial group C6 -R3C; Li 3v and Nb5+ occupy the equivalent rhombohedra1 vacancies with local C3 symmetry (1). On analysing optical spectra we find that some lines have a distinct two-component structure, which is practically independent of the activator concentration. Fig. 1 shows the nonpolarized absorption and luminescence spectra in the axial direction 3+ of a LiNb03 - Nd crystal for activator concentrations 0.003 and 5.0 wt% (to the initial charge).

From this figure it i s evident that the two-component lines a r e

distinctly observed in the low-temperature spectra associated with the transitions 4 4 4 Such a bifurcation i s observed 1g/2i2p1/29 'g/74F3/27and F 3 / 2 - '11/2* also on the lines of other groups with inter-component interval not exceeding -1 On increasing the activator concentration new lines appear in absorption 10 cm

.

spectra, and their intensities steeply rise, for instance, in our example these are 4 4 the lines a and c (transition 19/2-2P1/2) and b (transition I9 / T F3/2)* Besides, this figure shows the concentration plots of absorption coefficients of .'

1.fe

' z , :w lines, and for the sake of comparison, similar plots a r e also shown for

srme of the lines which can be recorded at any arbitrary activator concentration.

The plots corresponding to the lines a

,

b , c and 1, 2, 3 form two families of

curves with different concentration dependences. The luminescence spectra did not 1)According to (2) the distribution coefficient of Nd < 0.1.

3+

ions in LiNbO crystals i s 3

K38

physica status solidi (a) 3

I

I

~

CA B

4 4

4

.i

Fig. 1. Non-polarized absorption and luminescence spectra of Nd 3+ ions in LiNbO

3

-

crystals for activator concentrations of 0.003 and 5.0 wt% (to the initial

-

charge) at 77 and300 0K: I-transition 4 F -+ 4 I 11-transition 4 I 11/2' 9/2 3/2 4F3/2,lII-transition 4I9/z-2P1/z, IV - concentrationdependence of absorption coefficients for the lines a , b, c and 1, 2, 3

exhibit any new lines within the concentration range used in the experiments. This is evident from Fig. 1 which presents the luminescence spectra associated with the 4

4

transition F From general crystallochemical considerations, we can 3/2'11/2* presuppose that on entering the LiNbOQlattice, the Nd 3+ ions a r e capable of replacing 1+ 5+ both Li , a s well as Nb ions in the equivalent vacancies (C ). When both Li and 3 Nb are replaced simultaneously, there arises the question of compensating the excess charge caused by the nonisovalency between Nd 3+ and Lil+ o r NbS ions. In such a case, the activator in LiNbO crystals will be formed by two structurally 3 similar centers. We assumed that the bifurcation of the spectral lines is due to

Short Notes

K39

Table 1 Energy levels of Nd 3+ ions in LiNbOQ Terms

Nu

--T

at 77 ' ~ ( c m - l ) ~

4~ 9/2 4, ,41/2 '13/2 4, '1 5/2 4F 3/2 F5/2' 2H9/2

be

k

sb -wi 0

AE cm

-1

0,156,170,440,486

5

5

486

1987,2033,2107,2190,2228,2263

6

6

276

3918,3973,4035,4118,4140,4184,4211

7

7

293

5777,5916,6005,6087,6105,6217,6290,6449

8

8

67 2

11250,11409

2

2

159

12133*, 12291,12396,12421,12449,12463,12574, 12692*

8

8

558

13199,13291,13398*, 13437

6

4

238*

14498,14567,14620,14664,14685

5

5

187

15748,15803,15873,15888,15911,15926

6

6

178

16753,16852,16909

3

3

156

16!M9*, 17071,17135,17176

4

4

227*

18719,18786,18836

4

3

117*

19135,19260,19331

5

3

196*

20682,20764,20799,20868,20938

5

5

256

14

7

7 23*

22914

1

1

-

23348,23554,23608

3

3

260

25745,25882

2

2

137

27365,27469

2

2

104

1

-

20973,21022,21039,21213,21372,21631,21696

28031

-

1

-

Note: Energy levels and AE values indicated by asterisks require more accurate definition. this phenomenon. Two such structurally similar centers were observed in paper (3) at low activator concentrations in the EPR studies of Nd 3+ ions in LiNb03 crystals.

physica status solidi (a) 3

K40

Table 2 Terms

3+ Energy levels of Er ions in LiNbO 3 at 77 0K(cm-')

Number 8 @

Ab w

AE (cm-l)

- 4 :15/2

4'13/2

0,63,132,156*,182,278,353,414

8

8

414

6524,6586,6611,6631,6759,6778,6804

7

7

280

51/2 41 9/2

10207,1026$, 10270,10303,10316,10338

6

6

131

12381 12410,12453,12603,12612

5

5

231

4F

15151,15174*,15255,15344

5

4

193.

4s3/2

18265,18354

2

2

89

19043*,1906l*,19152*,19177*,1918s*,19199

6

6

156*

20970,20407*,20538,20557

4

4

187

22060,22124,22136

3

3

76

4F

22395,22526

2

2

131

2H

24371*,24417,24444,24500,24594

5

5

223*

6

6

213*

9/2

2H11,2

4F

7/2

F5 /2

3/2 9/2

G11/2

26185*,26197,26300,26349,26380,26398

--

Note: Energy levels and AE values indicated by asterisks require more accurate definition. In analyzing the spectra and in constructing a scheme for the crystalline splitting

of levels, we regarded these almost similar activator centers as one llquasi-centerfl. 3+ The energy levels of Nd ions in LiNb03 crystals at 77 OK are shown in Table 1. The experiments on the absorption, luminescence and stimulated emission of 3+ LiNb03 - Nd made it possible to determine with great reliability the position of 4 the I multiplet terms, whose crystalline Splitting is shown in Fig. 2. The dotted J arrows indicate the transitims corresponding to the faint lines in the absorption spectra. As regards the new lines appearingat highconcentrations, we can attribute them to paired or more complex associates with respect to the nature of the concentration dependence of their absorption coefficients. Analogous phenomena were observed with LiNbO crystals activated by Er* ions. An analysis of the 3

Short Notes

K41

I I I

I I

I I

I I I I

I I

I 2

4

Fig. 2. The scheme of crjrstalline splitting of terms P and I of 1/2' F3/2' J 3+ Nd ions in LiNbO crystals at 77 OK. The position of levels a r e given 3 -1 in cm , and the transition between them in 8 . The stimulated transitions

A , B, and C are denoted by thick arrows

optical spectra gave the exact position of levels of all those terms, transitions between which were observed in the transparent band of the crystal (0.35 to 5.5 ym). These results a r e shown in Table 2. Fig. 3 shows a scheme for the energy levels 3+ ions in LiNb03 which a r e directly associated with the observed luminescence. of E r 3+ 3+ Thus, at low concentrations of activator ions (Nd , E r ) in LiNbOg crystal, two structurally similar centers are formed which are associated with equiprobable 1+

substitutions of Li

and Nb* ions. At higher admixture concentrations in LiNbO

paired or more complex associates a r e formed.

3'

physica status solidi (a) 3

K42

88 I

I I I

I I

I I I I I

I I I I

4 4 Fig. 3. The scheme of crystalline splitting of levels of terms S3/2, I13/2' and 3+ 0 of E r ions in LiNbO crystal at 77 K. The position of levels 3 1 ' 5/2 -1 is given in c m , whereas the transitions between them in %

References

(I) S.C. ABRACHAMS, J. M. REDDY, and J. L. BERNSTEIN, J. phys. Chem.

a,

997 (1966). (2) K. NASSAU, Ferroelectricity, Coll. Papers 259, Amsterdam 1967. (3) N. F. EVLANOVA, L.S. KORNENKO, L. N. RASHKOVICH, and A.O. RYBALTOVSKII, Zh. eksper. teor. Fiz. 5 3 -

1920 (1967).

(Received July 31, 1970)

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